Radio communication systems may have a cellular architecture, with each cell corresponding roughly to a geographical area. Each cell includes a base station (BS), which is a local central cite through which a number of radio transmitter/receiver units (user terminals (UTs)) gain access to the communications system. The UTs could be, for example, telephones, PDAs, or small modem boards. A UT establishes a communication link with other UTs by requesting access to the system through the BS. Each UT communicates over a communication channel distinguished from other UTs.
Various techniques exist to increase the number of available channels for a given number of available frequencies. Time division multiple access (TDMA), for example, divides a single frequency into multiple time slots. Each of the time slots can then be allocated to a separate communication channel. Other known techniques include code division multiple access (CDMA) and frequency division multiple access (FDMA), which, like TDMA, are considered conventional multiple access schemes.
Radio communications systems may employ a spatial division multiple access (SDMA) scheme in conjunction with one or more conventional multiple access schemes to increase the number of UTs that a BS can serve for a given number of available frequencies. A SDMA scheme may be implemented using a BS that has an array of receiver antenna elements. The antenna elements are spaced, one from another, typically about a half of a meter apart. The array of antenna elements introduces multiple versions of the signal received from a UT. Each of these versions includes co-channel interference and noise. The co-channel interference is the result of multiple UTs attempting to randomly access the system on the same channel at the same time. Due to the spacing of the antenna elements, the amplitude and phase of a signal from a particular UT relative to the interfering UTs will be different for each of the multiple versions of the signal. By appropriately processing the multiple versions of the signal, it is possible to spatially determine multiple signals on the same communication channel, thereby increasing the number of UTs that can be served on a particular channel. To do this requires the ability to separate a particular signal from a number of signals from UTs attempting to communicate on the same channel at the same time. A number of methods are available for concurrent separation and successful resolution of such colliding signals. This may be done by determining one or more signal parameters that differ from one signal to another. The signal parameters are used to differentiate between the multiple colliding signals. Such parameters may include timing offset, frequency offset, spatial signatures and training data. The number of signals that may be recovered depends on the method, but generally, resolving more concurrent signals requires more computation. If the number of concurrent signals exceeds the receiver's capabilities, some of the signals are lost.
For example, a system that employs SDMA in conjunction with a TDMA scheme using a multi-antenna element receiver may use training sequences to determine a set of spatial weights to be applied to the signal received at each of the respective antenna elements. The training sequences, which allow transmissions from different UTs to be distinguished, are known as priori at the BS receiver. When the training sequence is received from a particular UT, the processing at the receiver attempts to distort the received signal such that it resembles the known training sequence. Spatial weights are determined and applied to the received signal to minimize the difference (error) between the received signal and the training sequence for a given timing offset. The training sequence is shifted over a set of different timing offsets within a timing window in order to minimize the error. This provides spatial weights and a course timing offset for each signal. That is the signal having the timing offset that minimizes the error is determined to be the signal from the particular UT, and spatial processing is used to extract that UT's signal. Typically, if two or more UTs are attempting to access the system on the same channel (having the same training sequence), having different timing offsets within the same timing window, the system will allow access to the strongest signal and discard the others. The UTs whose signals were discarded attempt access to the system after some delay. Alternatively, if it is desired to receive two, or more of the signals, the signals must be processed in parallel on the same digital signal processor (DSP). The receiver DSP may simply not have enough processing power, and it may not be economical or efficient to equip a BS receiver with a DSP having the required processing power. Typically a BS receiver has multiple DSPs with lower processing power.
A good signal resolution method should be both computationally efficient and scalable. A method is “scalable” if it can be easily configured to recover signals from more UTs when it is given more capable hardware. That is, the algorithm should be able to determine the parameter values of a greater number of communication signals given greater processing power.